Field of the Invention and Related Art Statement
The present invention relates to an oxygen sensor, and more particularly to a waterproof type oxygen sensor for measuring an oxygen concentration of exhaust gases discharged from an automobile. The oxygen sensor comprises an oxygen sensing element including inner and outer electrodes which are to be contacted with the atmospheric air and the exhaust gases, respectively, a tube-like metal cap for accommodating the oxygen sensing element, at least two gas-tightly sealing sections provided in the metal cap for isolating a reference gas space into which the inner electrode of the oxygen sensing element is exposed from the exhaust gases, and a communicating section for communicating the reference gas space with the surrounding atmospheric air.
The waterproof type oxygen sensor of the type mentioned in the preamble has been widely known as a detector for measuring an oxygen concentration in gases exhausted by an automobile. In the oxygen sensor of this type, the atmospheric air is generally used as a reference oxygen, and therefore the sensor is constructed to isolate the reference gas space into which the atmospheric air is introduced from the exhaust gases to be measured.
FIG. 1 is a half cross sectional view showing a known non-waterproof type oxygen sensor. The known oxygen sensor 2 shown in FIG. 1 comprises a metal housing 4 which accommodates a plate-shaped oxygen sensing element 6 and a metal cap 8. In order to separate the atmospheric air from the exhaust gases, in a space between the plate-like sensing element 6 and the metal cap 8 is filled with talc 10 which constitute gas-tight sealing sections. In this manner, an inner electrode of the oxygen sensing element 6 is isolated from the exhaust gases to be measured. At the same time, there is formed an air introduction passage for introducing the atmospheric air into the reference oxygen space within the oxygen sensor. In FIG. 1 this passage is constructed by an air communication opening 12 formed in the metal cap 8.
FIG. 2 illustrates another known non-waterproof type oxygen sensor 2 in which use is made of a test tube-shaped oxygen sensing element 14. In this known oxygen sensor, the above mentioned air introducing passage is formed by a space between the metal cap 8 and a metal boot 16 applied on the metal cap.
In the known oxygen sensors so far explained, when water intrudes into the reference gas space of the oxygen sensor, the water is vaporized to reduce a partial pressure of the reference oxygen, so that an electromotive force is decreased. In order to avoid such intrusion of water, various waterproof constructions have been prepared. For instance, an inner space between a stranded lead wire is used as the air introducing passage and a distal end of the passage which is provided at such a location that the water is not applied thereto is opened into the atmospheric air. Further, there is provided a water-repelling gap between the metal cap and the metal boot or a water-repelling porous member provided at an inlet opening of the air introducing passage.
However, it has been experimentally confirmed that when the known waterproof oxygen sensors are used for a long-term usage under high load-running conditions, the electromotive forces are decreased. As will be explained later in detail the inventors have found that the above mentioned decrease in the electromotive force is mainly caused by the intrusion of the exhaust gas into the reference gas space via the gas-tightly sealing section .